What can be done about obsessive humping in dogs?

Get him fixed. If he already is, then it's either playing or showing a sign of dominance. If simply yelling "NO! " or proving you're the pack leader doesn't curtail the behavior, leave the room. Being ignored may stop the behavior. He may also have an anal sac infection, so you could have the vet check that.

This is a natural dominance behavior, not sexual as you might think. If they are doing it to other dogs, it is completely normal and should not be stopped, as dogs need to establish order in the pack if more than one is living together. If they are humping you, then change your training and establish yourself as pack leader.

Sometimes having your dog fixed will remove this behaviour. If it doesn't then you can speak to your Veterinarian about hormones and behavioral deterents. Some options include anything from hormone supplements to negative reinforcement.

Neutering may be the way to go. Far from being cruel, it keeps dogs who have little contact with other dogs from becoming frustrated in their isolation. Dogs are naturally pack animals, and as such, mated often. Most domestic dogs do not get this opportunity, so it is humane to turn their hormone levels down a bit.